从 R 中的列表中提取数据帧

Question

I am trying to extract individual counties as a dataframe from a list (which consists of data from all the counties) as separate data frames in R. My code is as below and for the sake of illustration, I am breaking into step 1 (extract data from URL to a list - this part works well) and step 2 (extract individual data.frames from list - this is not working well and gives an individual list with only the last list item)

## Step 1: Extract data from URL 
library(data.table)

# List of counties (just a sample here)
x <- data.frame(county = c("12001", "12003", "12005"))

idx <- x$county

#Extract data from URL for list of counties
  qcew_q1 <- lapply((1:nrow(x)),function(area) {
  url <- "http://data.bls.gov/cew/data/api/YEAR/QTR/area/AREA.csv"
  url <- sub("YEAR", 2020, url, ignore.case=FALSE)
  url <- sub("QTR", 1, url, ignore.case=FALSE)
  url <- sub("AREA",idx[area] , url, ignore.case=FALSE)
  fread(url, header = TRUE, sep = ",", quote="\"", dec=".", na.strings="", skip=0)
  
}
)

Once I extract the data from the URL to a list, I am trying to extract the individual counties as separate data frames. This the part that is causing issues where it gives only the last item and writes it to a list instead of a data.frame. Any insights would be much appreciated.

## Step 2: Extract data from step 1 as separate data frames. 
## Writes only last list (12005) to another list.

#Using For statement
for(c in 1:nrow(x)){
  for(i in 1:3){
  q1_idx[c] <- qcew_q1[i]
}
}

# Using lapply
lapply(1:nrow(x),function(cnty){
  for(i in 1:3){
    q1_idx[cnty] <- qcew_q1[i]
  
  }
})

Any insights on how to fix this would be much appreciated.

TIA,

Krishnan

Answer 1

You have list of dataframes in qcew_q1 and it is almost always better to keep them as such if you want to perform any further analysis on it. It is easier to manage and does not pollute the global environment. Maybe to clarify which dataframe is from which county you can assign them names.

names(qcew_q1) <- x$county

If you want a specific county dataframe you can extract it as qcew_q1[['12001']] or qcew_q1[['12003']] . You can use lapply to iterate over the list and apply a function to each individual dataframe.

If you still want individual dataframes in the global environment, assign them names as per your choice and use list2env .

names(qcew_q1) <- paste0('county_', x$county)
list2env(qcew_q1, .GlobalEnv)

The individual dataframes are now called county_12001 , county_12003 etc.